AC motors in comparison to DC motors are generally of simpler structure and include the advantages of lower cost, more compact size, less weight and better operation at higher speeds and higher efficiencies. However, controllers for AC motors have been complex and expensive, hampering the adoption of AC motors for use with DC supplies such as motor vehicle batteries.
One system useful for the regulation of motor currents is the current regulating pulse-width modulated (CRPWM) inverter. The inverter typically utilizes analog comparators and summing amplifiers to process a computer-generated reference current command for each phase of the motor in such a manner as to determine the states of upper and lower phase switches in the power stage of the inverter. Simplification of AC motor controllers has resulted from the use of such current-mode controllers.
For current-mode control, the current flowing into the motor is measured and compared to reference current signals which are generated by a motor controller and are representative of desired current for the motor to effect desired operating conditions. Error signals resulting from the comparison are used to switch semiconductor elements of an inverter circuit which provides power to the motor. Pulse-width modulated DC power is thus rapidly switched to the motor terminals.
One popular form of current-mode control utilizes hysteresis comparators, wherein the actual currents within a motor are maintained within a band of given amplitude centered on reference currents which are representative of desired current levels within the motor. An example of hysteresis current-mode control is disclosed in U.S. Pat. No. 5,027,048, issued to Masrur et al. on Jun. 25, 1991, which is assigned to the assignee of the present invention and is hereby incorporated by reference. The hysteresis motor controller of the Masrur et al. patent also discloses the use of field-oriented control, which is another important improvement in the control of AC motors.
Referring now to FIG. 1, a schematic diagram of a current-mode controller is shown generally by reference numeral 10. The current-mode controller 10 is for use in the current-mode hysteresis controller disclosed in the above-referenced pending application, U.S. Ser. No. 07/787,805, filed Nov. 4, 1991. As shown in FIG. 1, the comparators 12 each include first comparator means including an operational amplifier 14 which sums or compares a desired inverter output current (I.sub..phi..sup..) for a particular phase with a sensed inverter output current (I.sub..phi.), to generate error signals representative of the difference therebetween.
The error signals from the amplifier 14 are passed to current band control means including an amplifier 16, which amplifies the error signals by an amplification factor determined by resistors 18, 20 and 22. The upper and lower signal limits of the hysteresis bands for the amplified error signals are determined by second comparator means including a comparator 24. The amplified error signals are compared to the fixed upper and lower signal limits to define at least two hysteresis bands. The amplification factor of the amplifier 16 is selectively controlled by means of a transistor 26 which is controlled via a hysteresis switch (HSW1) signal applied through a resistor 28.
For the case of two hysteresis bands, the amplification factor of the amplifier 16 is selected based on the desired inverter output current I.sub..phi..sup.. for the particular phase or the reference current for that phase. The selection in this case can be performed by a comparator 30 which sums or compares the desired inverter output current I.sub..phi..sup.. with a signal representative of a desired reference current I.sub.REF. Thus, the transistor 26 determines whether the resistor 22 is connected into the gain control circuit for the amplifier 16 or shorted out and not included in determining the gain. For operation with more than two hysteresis bands, additional control resistors 32 and corresponding control transistors 34 can be provided and controlled by a corresponding hysteresis switch (HSW2-HSWX) signal to define a series of gains for the amplifier 16.
With continued reference to FIG. 1, the upper and lower limits to which the amplified error signals are compared are substantially fixed for a given application. However, the limits can be selected within defined ranges. The upper limit for the comparison to the amplified error signal is performed by the amplifier 24 as defined by the setting of a potentiometer 36. The lower limit for the comparison to the amplified error signals is performed by the amplifier 24 as defined by the setting at a potentiometer 38 in cooperation with an amplifier 40 and the precision reference voltage V.sub.REF. The potentiometer 36 defines the width of the hysteresis window and the potentiometer 38 and precision reference voltage V.sub.REF adjusts the symmetry of the hysteresis window.
The method of operating the current-mode hysteresis controller 10 for controlling a pulse-width modulated inverter in response to desired and sensed inverter output currents includes the steps of sensing the output current in a pulse-width modulated inverter circuit to be controlled and generating a reference current representative of the desired current for the sensed motor phase. The method also includes the steps of comparing the sensed output current to the reference current to generate error signals representative of the differences therebetween and generating an inverter current switch signal whenever the error signals exceed a selected hysteresis band surrounding the reference current. The method also includes the steps of defining at least two hysteresis bands surrounding the reference current and corresponding to different reference current magnitudes and selecting one of the hysteresis bands.
These systems make use of analog circuitry. Component characteristic variations due to temperature effects can cause analog circuit outputs to drift somewhat over time. Analog drift introduces DC components into the motor current, thereby adding errors to the current regulation and control of motor torque, position and/or speed.
Accordingly, there is a need for an improved control system and method for substantially reducing errors associated with the use of analog hardware for current regulation while still providing for high efficiency, high accuracy system operation.